In pipeline operation, starting and stopping a pump or opening or closing a valve will produce a surge in pressure which, under certain circumstances, could build up to dangerous proportions and may cause pipeline damage. In some pipeline installations, valves are provided which open at a predetermined, dangerously high pressure. However, such valves are necessarily installed at locations along the pipeline where surges are likely to be of greatest intensity, e.g., at the bottom of a steep grade, and not necessarily at locations convenient for inspection and service. Hence, it is desirable to protect against a pressure surge in a pipeline when pressure increases at too rapid a rate, even though pressure at the particular point in the pipeline has not reached an excessive level.
In one system, pipeline surges are relieved by dumping the pipeline fluid to a reservoir. A pressure-responsive valve in the dump line opens automatically when the pressure in the dump line, i.e., pipeline pressure, overcomes the pressure of gas in a pilot chamber. The pilot chamber is normally at pipeline pressure, but an orifice in the pilot load line retards pressure build-up in the event of a surge, producing a temporary imbalance between line pressure and pilot pressure, allowing line pressure to open the valve.
An accumulator, including a diaphragm, a bladder or similar device is installed in the pilot load line to maintain the pilot gas at the pressure of the pipeline liquid in the pilot load line. However, the diaphragm, the bladder or other medium for transferring pressure from pipeline liquid to gas may be susceptible to the corrosive properties of some liquids, such as crude oil.